Two-stage screw compressor

ABSTRACT

A two-stage screw compressor of the invention includes a low pressure stage casing provided with a low pressure stage compression mechanism, a discharge casing provided on a discharge side of the low pressure stage casing, a high pressure stage casing provided with a high pressure stage compression mechanism, and intermediate stage casings for connecting the discharge casing and the high pressure stage casing, and accommodating an electric motor. In such a two-stage screw compressor, a cavity is formed on a discharge casing side in the slide valve so as to enlarge the capacity of a discharge space formed in the discharge casing. Thereby, internal resonance occurring in the discharge casing is prevented.

INCORPORATION BY REFERENCE

The present application claims priority from Japanese application JP2005-190879 filed on Jun. 30, 2005, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to a two-stage screw compressor used for ascrew refrigerator or the like.

Regarding a noise spectrum generated by a screw compressor, it is knownthat a fundamental frequency obtained by multiplying the number of teethof a male rotor by a rotational frequency, and a higher harmoniccomponent of the fundamental frequency constitute a dominant peak. Asmeans of reducing the noise, various measures such as sticking a noiseabsorbing material to a casing, enhancing sound insulation performanceby forming a casing to have a double structure, or enhancing rigidity ofa casing by adding a rib thereto have been taken. As a single-stagesingle screw compressor, there is the one in which a resonance space isprovided inside a slide valve so as to communicate with a compressionspace to reduce the noise, as shown in JP-A-2005-30362.

In the above described prior arts, the measures according to the meanssuch as sticking the noise absorbing material to the casing, forming thecasing into the double structure, and adding the rib to the casing showa certain degree of noise reduction effect. However, if adopting theabove described means, it is necessary to cover the entire cast casingwith the noise absorbing material in the case of sticking the noiseabsorbing material on the casing, and thus it is often difficult in thecase of a screw compressor having a complicated cast shape. Further, inthe case of the means according to the double structure of the casingand the rib addition, there are the problems of complication of the castshape and increase in the cast weight.

In addition, any of the above described prior arts does not take accountof increase in noise due to internal resonance in a discharge casing ina two-stage screw compressor, which includes a low pressure stage casingprovided with a low pressure stage compression mechanism, the dischargecasing provided on a discharge side of the low pressure stage casing, ahigh pressure stage casing provided with a high pressure stagecompression mechanism, and an intermediate stage casing for connectingthe discharge casing and the high pressure stage casing andaccommodating an electric motor.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to prevent internal resonanceoccurring in a discharge casing by means of a simple structure, andthereby realize reduction in noise, in a two-stage screw compressor asdescribed above.

In order to achieve the above-described object, the present inventionprovides a two-stage screw compressor including: a low pressure stagecasing for accommodating a low pressure stage compression mechanism anda bearing member, which low pressure stage casing is provided with acapacity control part constituted by a slide valve, a rod and a piston;a discharge casing provided on a discharge side of the aforesaid lowpressure stage casing for accommodating a bearing member supporting thelow pressure stage compression mechanism; a high pressure stage casingfor accommodating a high pressure stage compression mechanism and abearing member; and an intermediate stage casing for connecting theaforesaid discharge casing and the aforesaid high pressure stage casing,and accommodating an electric motor, wherein a cavity is formed on adischarge casing side in the slide valve so as to enlarge the capacityof a discharge space formed in the aforesaid discharge casing, so thatinternal resonance occurring in the discharge casing is prevented.

In this case, by making the length in an axial direction of the cavityformed in the slide valve half or more of the length in an axialdirection of the slide valve, it becomes possible to prevent theinternal resonance more reliably.

Further, it is more effective to provide a porous material in the cavityformed in the slide valve. As the porous material, it is suitable to usea demister constructed by arranging a plurality of metal wires in a netform. Alternatively, the porous material may be constructed by glasswool.

Another characteristic of the present invention is, in the abovedescribed two-stage screw compressor, that the length in an axialdirection of a discharge space formed in the aforesaid discharge casingis enlarged by forming the cavity on the discharge casing side in theslide valve so as not to correspond to a wavelength of noise generatedby the low pressure stage compression mechanism. In this case,preferably, the wavelength of the noise generated by the low pressurestage compression mechanism is based on a meshing frequency of rotors,which meshing frequency is a frequency including a fundamental frequencyobtained by multiplying the number of teeth of a male rotor by arotational frequency, and a higher harmonic component of the fundamentalfrequency, and it is desirable to enlarge the length in the axialdirection of the discharge space formed in the aforesaid dischargecasing by forming the cavity on the discharge casing side in the slidevalve so as not to correspond to any of the wavelengths based on theabove described frequencies.

According to the present invention, in a two-stage screw compressorincluding a low pressure stage casing provided with a low pressure stagecompression mechanism, a discharge casing provided on a discharge sideof the low pressure stage casing, a high pressure stage casing providedwith a high pressure stage compression mechanism, and intermediate stagecasings for connecting the discharge casing and the high pressure stagecasing, and accommodating an electric motor, a cavity is formed on adischarge casing side in the slide valve to enlarge the capacity of adischarge space formed in the discharge casing, and thereby internalresonance occurring in the discharge casing is prevented. Therefore, theinternal resonance occurring in the discharge casing is prevented with asimple structure and significant noise reduction can be realized.

Hereinafter, an embodiment of the present invention will be describedbased on the drawings.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a vertical sectional view of a two-stage screw compressorshowing one embodiment of the present invention;

FIG. 2 is a vertical sectional view of a main part of a two-stage screwcompressor showing another embodiment of the present invention; and

FIG. 3 is a noise spectrum diagram for explaining the effect of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows one embodiment of a two-stage screw compressor of thepresent invention. The screw compressor includes a low pressure stagecasing 1 having an inlet opening 26, a discharge casing 2 connected to adischarge side of the low pressure stage casing 1 and having a gaspassage 38, a motor cover (intermediate stage casing) 3 which isconnected to a downstream side of the discharge casing 2, has a gaspassage 39, and accommodates a coil end 8 of an electric motor 7, amotor casing (intermediate stage casing) 4 accommodating the electricmotor 7, and a high pressure stage casing 5 having a discharge port 40.These members are connected with bolts or the like to be integrated.

In the low pressure stage casing 1, there are formed a cylindrical bore31, an inlet port 29 which introduces a refrigerant gas into thecylindrical bore 31, a piston chamber 33 which accommodates a piston 21for driving a slide valve 23, and the like. In the cylindrical bore 31,a low pressure stage side male rotor 6 which is rotatably supported withroller bearings 11 and 12 and a ball bearing 17, and a low pressurestage side female rotor (not shown) are accommodated so as to mesh witheach other. A shaft of the low pressure stage side male rotor 6 isconnected to a shaft of the high pressure stage side male rotor 9 by agear coupling 45. The piston 21 accommodated in the above describedpiston chamber 33 is connected to the slide valve 23 via a rod 22.

In the discharge casing 2 accommodating the roller bearing 12 and theball bearing 17 which support the shaft of the low pressure stage siderotor, there is formed a gas passage 38 which allows the cylindricalbore 31 and the above described motor cover 3 to communicate with eachother. In the motor cover 3 accommodating a ball bearing 18 whichsupports a rotary body on the high pressure stage side, there is formeda gas passage 39 which allows the discharge casing 2 and the motorcasing 4 to communicate with each other. In the motor casing 4accommodating the electric motor 7 and a shaft part of the high pressurestage side male rotor 9, there is formed an inlet chamber 28 whichintroduces the refrigerant gas to the high pressure stage casing 5. Inthe high pressure stage casing 5, there is formed a cylindrical bore 32,in which the high pressure stage side male rotor 9 which is rotatablysupported with roller bearings 13 and 14 and a ball bearing 19, and ahigh pressure stage side female rotor (not shown) are accommodated so asto mesh with each other. The shaft of the high pressure stage side malerotor 9 is directly connected to the electric motor 7.

Next, flows of the refrigerant gas and oil will be described. Therefrigerant gas at low temperature and low pressure which has beensucked from the inlet opening 26 provided in the low pressure stagecasing 1 is sucked from the intake port 29 formed in the low pressurestage casing 1 into a compression chamber formed by surfaces of themeshing teeth of the male and female screw rotors on the low pressurestage side and the low pressure stage casing 1. Thereafter, therefrigerant gas is sealed in the compression chamber formed by themeshing tooth surfaces of the male and female screw rotors on the lowpressure stage side and the low pressure stage casing 1 as the lowpressure stage side male rotor 6 connected to the electric motor 7 isrotated, and is gradually compressed by reduction of the compressionchamber to become a gas at high temperature and at high pressure, whichis discharged from the discharge port 34 to the gas passage 38 of thedischarge casing 2 and further, passes through the gas passage 39 formedin the motor cover 3 to flow into the motor casing 4.

The refrigerant gas having flown into the motor casing 4 passes throughan air gap between motor rotors, and is sucked from the intake chamber28 formed in the motor casing 4 to a compression chamber formed bysurfaces of the meshing teeth of the male and female screw rotors on thehigh pressure stage side, the motor casing 4, and the high pressurestage casing 5. Thereafter, the refrigerant gas is sealed in thecompression chamber formed by the meshing tooth surfaces of the male andfemale screw rotors on the high pressure stage side, the motor casing 4,and the high pressure stage casing 5 as the high pressure stage sidemale rotor 9 directly connected to the electric motor 7 is rotated, andis gradually compressed by reduction in the compression chamber tobecome a gas at higher temperature and at higher pressure than beforebeing compressed, which is discharged into the discharge passage 40 ofthe high pressure side casing 5.

The radial load, derived from compression reaction force acting on themale and female screw rotors on the low pressure stage side duringcompression, is supported by the roller bearings 11 and 12, and thethrust load derived from the compression reaction force is supported bythe ball bearing 17. As for compression reaction force acting on themale and female screw rotors on the high pressure stage side duringcompression, the radial load is supported by the roller bearings 13 and14, and the thrust load is supported by the ball bearing 19. Lubricatingoil for these bearings is supplied through an oil passage communicatingwith bearing parts provided in the low pressure stage casing 1, thedischarge casing 2, the motor cover 3, the motor casing 4 and the highpressure stage casing 5, respectively.

In a noise spectrum generated by a screw compressor, a fundamentalfrequency obtained by multiplying the number of teeth of a male rotor bya rotational frequency, and a higher harmonic component (hereinafter,referred to as a meshing frequency) of the fundamental frequencyconstitute a dominant peak. In the case of the two-stage screwcompressor, the discharge casing 2 and the intermediate stage casings(the motor cover 3, the motor casing 4) are required for connecting thelow pressure stage casing 1 on the low pressure stage side and the highpressure stage casing 5 on the high pressure stage side. The gas passage38 and the gas passage 39 are formed in the discharge casing 2 and theintermediate stage casing (motor cover 3), respectively, as spaces. Ifthe length of these spaces 38 and 39 and the wavelength of the meshingfrequency correspond to each other, a noise level increases due tointernal resonance.

In the present invention, in order to enlarge the capacity of the spacesformed as the above described gas passages 38 and 39, namely, of adischarge space so as not to correspond to the wavelength of the abovedescribed meshing frequency, a cavity 48 is formed on a discharge casingside (gas passage side) in the slide valve 23. By forming the cavity 48,it becomes possible to avoid the internal resonance in the dischargespace (gas passages 38 and 39). That is, if the length in an axialdirection of the discharge space corresponds to the wavelength of themeshing frequency of the rotors, the internal resonance occurs toincrease the noise level, however, by providing the above describedcavity 48, the length in the axial direction of the discharge space canbe extended so as not to correspond to the wavelength of the meshingfrequency of the rotors. As a result, it is possible to avoid theinternal resonance by a simple structure, in which the cavity 48 isformed in the slide valve 23, and to achieve reduction in noise. Thelength in the axial direction of the cavity 48 formed in the slide valve23 may be extended to half or more of the length in an axial directionof the slide valve, and thereby, the effect of reliably preventing theinternal resonance can be obtained.

FIG. 2 shows another embodiment of the present invention. In thisembodiment, a porous material 49 is provided in the cavity 48 formed inthe slide valve 23. By providing the porous material 49 like this, itbecomes possible, not only to avoid the internal resonance by the cavity48, but also to convert acoustic wave energy into thermal energy byinternal friction of the porous material 49, and thus the noisereduction effect can be further enhanced. The porous material 49 may beanything as long as it has an oil resistance property and a refrigerantresistance property, but extremely high noise reduction effect can beobtained by adopting a demister or glass wool. The demister in this casemeans the one which is given the function as a porous material byarranging a plurality of metal wires in a meshed form.

FIG. 3 is a noise spectrum diagram for explaining the effect of thepresent invention. When there is no cavity in the slide valve 23, thelength in the axial direction of the space formed as the gas passages 38and 39, and the wavelength of a fifth order component (5 f) of themeshing frequency correspond to each other, and thus the fifth ordercomponent (5 f) becomes a dominant peak due to a resonance phenomenon.On the contrary, according to the present invention, since the cavity 48is provided in the slide valve 23, the space formed as the gas passages38 and 39 can be enlarged. As a result, by applying the presentinvention, the length in the axial direction of the entire enlargedspace does not correspond to the wavelength of the fifth order component(5 f) of the meshing frequency, and therefore, it becomes possible toavoid the resonance phenomenon. Accordingly, it becomes possible toreduce the peak of the fifth order component (5 f) significantly, sothat significant noise reduction effect can be obtained.

The noise data shown in FIG. 3 is only one example. The peak which canbe decreased by applying the present invention depends on the length inthe axial direction of the discharge space and on the wavelength of themeshing frequency, and thus the fifth order component (5 f) does notalways become the dominant peak. According to the present invention,whatever order component constitutes the dominant peak, it is possibleto extend the length in the axial direction of the discharge space byforming the cavity 48 in the slide valve 23, so as not to correspond tothe wavelength of the frequency component at which the resonance occurs,which can reduce noise.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A two-stage screw compressor, comprising: a low pressure stage casingfor accommodating a low pressure stage compression mechanism and abearing member, the low pressure stage casing being provided with acapacity control part which is constituted by a slide valve, a rod and apiston; a discharge casing provided on a discharge side of said lowpressure stage casing for accommodating a bearing member supporting thelow pressure stage compression mechanism; a high pressure stage casingfor accommodating a high pressure stage compression mechanism and abearing member; and an intermediate stage casing for connecting saiddischarge casing and said high pressure stage casing, the intermediatestage casing accommodating an electric motor, wherein a cavity is formedon a discharge casing side in the slide valve so as to enlarge thecapacity of a discharge space formed in said discharge casing.
 2. Thetwo-stage screw compressor according to claim 1, wherein the length inan axial direction of the cavity formed in the slide valve is half ormore of the length in an axial direction of the slide valve.
 3. Thetwo-stage screw compressor according to claim 1, wherein a porousmaterial is provided in the cavity formed in the slide valve.
 4. Thetwo-stage screw compressor according to claim 3, wherein said porousmaterial is a demister constructed by arranging a plurality of metalwires in a net form.
 5. The two-stage screw compressor according toclaim 3, wherein said porous material is glass wool.
 6. A two-stagescrew compressor, comprising: a low pressure stage casing foraccommodating a low pressure stage compression mechanism and a bearingmember, the low pressure stage casing being provided with a capacitycontrol part which is constituted by a slide valve, a rod and a piston;a discharge casing provided on a discharge side of said low pressurestage casing for accommodating a bearing member supporting the lowpressure stage compression mechanism; a high pressure stage casing foraccommodating a high pressure stage compression mechanism and a bearingmember; and an intermediate stage casing for connecting said dischargecasing and said high pressure stage casing, the intermediate stagecasing accommodating an electric motor, wherein the length in an axialdirection of a discharge space formed in said discharge casing isenlarged by forming a cavity on a discharge casing side in the slidevalve so as not to correspond to a wavelength of noise generated by thelow pressure stage compression mechanism.
 7. The two-stage screwcompressor according to claim 6, wherein the wavelength of the noisegenerated by the low pressure stage compression mechanism is based on ameshing frequency of rotors.
 8. The two-stage screw compressor accordingto claim 7, wherein said meshing frequency is a frequency including afundamental frequency which is obtained by multiplying a number of teethof a male rotor by a rotational frequency, and a higher harmoniccomponent thereof.